The present invention relates to a MOS (metal oxide semiconductor) solid-state imaging device using MOS transistors for reading a signal and to a method of manufacturing the same.
Solid-state imaging devices are generally classified into two categories according to the method by which a signal is read: CCD devices, which use CCDs (charged coupled devices) to transfer signal charge and read a signal simultaneously from a plurality of pixels, and MOS devices, which use read-out circuits comprising MOS transistors, formed for each pixel, to read a signal by selecting one pixel after the other.
In recent years, MOS solid-state imaging devices, especially CMOS devices that are produced with a CMOS (complementary MOS) process, have received great attention as image input elements for portable imaging apparatus such as small PC cameras. Because they can be driven with low voltage and low power consumption, and they can be integrated on one chip together with peripheral circuits.
MOS solid-state imaging devices in turn are classified into two categories according to the read-out circuit that is formed for each pixel: passive devices, which directly read the signal charge that accumulates in a photo-receiving portion into an output line, and active devices, which amplify the potential difference that occurs due to the accumulation of the signal charge with an amplifying circuit before giving it out. FIGS. 9 and 10 are cross-sectional drawings showing structures of pixels in conventional MOS solid-state imaging devices. FIG. 9 shows a pixel in an active solid-state imaging device. The signal charge is transferred from the photo-receiving portion to a detecting portion. The potential difference occurring at the detecting portion is given out. Each pixel comprises a photo-receiving portion and four transistors: a charge transfer transistor, an amplify transistor, a reset transistor and a select transistor. The charge transfer transistor is a MOS transistor consisting of a photo-receiving portion 73a and a detecting portion 74a formed in a silicon substrate 70, an insulating film 71 formed on the silicon substrate, and a gate electrode 72 formed on the insulating film 71 at least between the photo-receiving portion 73a and the detecting portion 74a. The photo-receiving portion 73a corresponds to the source and the detecting portion 74a corresponds to the drain of the charge transfer transistor. FIG. 10 shows a pixel in an active solid-state imaging device giving out the potential difference occurring at the photo-receiving portion. Each pixel comprises a photo-receiving portion and three transistors: an amplify transistor, a reset transistor and a select transistor. The reset transistor is a MOS transistor consisting of a photo-receiving portion 83a and a charge drain portion 84a formed in a silicon substrate 80, an insulating film 81 formed on the silicon substrate, and a gate electrode 82 formed on the insulating film 81 at least between the photo-receiving portion 83a and the charge drain portion 84a. The photo-receiving portion 83a corresponds to the source and the charge drain portion 84a corresponds to the drain of the reset transistor.
As in regular MOS transistors, the MOS transistors in these MOS solid-state imaging devices have a lightly doped drain (LDD) structure, comprising a diffusion region with a low impurity concentration at the end of the drain region near the gate electrode (referred to as xe2x80x9cLDD portionxe2x80x9d in the following), to suppress deterioration of the element properties due to a concentration of the electric field near the drain. This LDD structure is also used in MOS transistors taking the photo-receiving portion for the source. For example, in the solid-state imaging device shown in FIG. 9, an LDD portion 74b is formed at the end of the detecting portion 74a, which corresponds to the drain of the charge transfer transistor, near the gate electrode. And in the solid-state imaging device shown in FIG. 10, an LDD portion 84b is formed at the end of the charge drain portion 84a, which corresponds to the drain of the reset transistor, near the gate electrode.
Furthermore, in a conventional MOS solid-state imaging device such as the one shown in FIG. 9 or FIG. 10, a diffusion region with a low impurity concentration (in FIG. 9, this is the region 73b, and in FIG. 10, this is the region 83b) is formed at the end of the photo-receiving portion near the gate electrode.
FIG. 11 illustrates a method for manufacturing a conventional solid-state imaging device with LDD structure such as the one shown in FIG. 9. First, impurity ions are implanted into the silicon substrate 70, whereon the insulating film 71 and the gate electrode 72 have been formed, to form the photo-receiving portion 73a, the LDD portions, and the detecting portion (FIG. 11(a)). However, at this stage the impurity concentration in the detecting portion is low and roughly equal to the impurity concentration in the LDD portions. Then, a silicon oxide film 75 is deposited (FIG. 11(b)). A portion of this silicon oxide film 75 is then removed by plasma etching (FIG. 11(c)). Silicon oxide films 75a and 75b remain on both sides of the gate electrode. Then, using the remaining silicon oxide films 75a and 75b as masks, ions are implanted again, to increase the impurity concentration in the photo-receiving portion 73a and the detecting portion 74a (FIG. 11(d)).
However, in solid-state imaging devices that are manufactured with the method explained above, the quality of the output image can deteriorate due to crystal defects in the photo-receiving portion, appearing as white marks for example.
It is an object of the present invention to provide a solid-state imaging device with superior quality of the output image and a method for manufacturing such a solid-state imaging device.
In order to achieve this object, a solid-state imaging device in accordance with the present invention comprises a plurality of pixels, each pixel comprising a semiconductor substrate of a first conductivity type; a photo-receiving portion of a second conductivity type formed in the semiconductor substrate; a first diffusion region of the second conductivity type formed in the semiconductor substrate; a first insulating film formed on the semiconductor substrate; a gate electrode formed on the first insulating film at least between the photo-receiving portion and the first diffusion region; a read-out circuit, which is electrically connected to one of the photo-receiving portion and the first diffusion region; and a second diffusion region of the second conductivity type formed in the semiconductor substrate, which is adjacent to an end of the first diffusion region near the gate electrode and separate from the photo-receiving portion. An impurity concentration in the photo-receiving portion and an impurity concentration in the second diffusion region are lower than an impurity concentration in the first diffusion region.
In this specification, the term of xe2x80x9cimpurity concentrationxe2x80x9d means the concentration of the impurities that give the desired conductivity characteristic to the region of the substrate where the impurities have been implanted.
In a conventional solid-state imaging device, the impurity concentration in the photo-receiving portion is equal to the impurity concentration in the drain region of the transistor having the photo-receiving portion as the source (in FIG. 9, this is the detecting portion 74a, and in FIG. 10, this is the charge drain portion 84a). Consequently, the photo-receiving portion may be damaged by the ion implantation for achieving such a high impurity concentration, which can deteriorate the quality of the output image.
On the other hand, in a solid-state imaging device according to the present invention, the impurity concentration in the photo-receiving portion is low, so that the damage inflicted upon the photo-receiving portion during the ion implantation for forming the photo-receiving portion is low, and white marks appearing on the output image can be suppressed. This improves the image quality of the output image.
It is preferable that the impurity concentration in the photo-receiving portion is lower than the impurity concentration in the second diffusion region. Because the impurity concentration in the photo-receiving portion of such a configuration is even lower, a good picture quality can be attained with more certainty.
It is preferable that each pixel further comprises a second insulating film formed on the first insulating film above the photo-receiving portion. It is even more preferable that the second insulating film covers the photo-receiving portion. However, if the photo-receiving portion has to be electrically connected to the read-out circuit, an aperture portion should be provided in the second insulating film through which the read-out circuit can contact the photo-receiving portion.
It is preferable that each pixel further comprises a third diffusion region of the first conductivity type formed at an upper portion of the photo-receiving portion in the semiconductor substrate. Since this reduces dark currents, a good picture quality can be attained with more certainty.
It is preferable that impurity concentration in the first diffusion region is at least 1020 cmxe2x88x923.
It is also preferable that the impurity concentration in the photo-receiving portion is 1015 cmxe2x88x923 to 1019 cmxe2x88x923.
It is also preferable that the impurity concentration in the second diffusion region is 1018 cmxe2x88x923 to 1019 cmxe2x88x923.
Moreover, it is preferable that the read-out circuit comprises an amplify transistor for amplifying an electrical signal corresponding to the light that is irradiated onto the photo-receiving portion. Thus, the output sensitivity of the signal can be increased.
An example for a pixel in such a solid-state imaging device is a pixel where the first diffusion region is electrically connected to the gate electrode of the amplify transistor. Another example for a pixel in such a solid-state imaging device is a pixel where the photo-receiving portion is electrically connected to the gate electrode of the amplify transistor and the first diffusion region is electrically connected to a terminal of a voltage source.
In a method for manufacturing a solid-state imaging device comprising a plurality of pixels in accordance with the present invention, wherein each pixel comprises a semiconductor substrate of a first conductivity type; a photo-receiving portion of a second conductivity type formed in the semiconductor substrate; a first diffusion region of the second conductivity type formed in the semiconductor substrate; a first insulating film formed on the semiconductor substrate; a gate electrode formed on the first insulating film at least between the photo-receiving portion and the first diffusion region; a read-out circuit, which is electrically connected to one of the photo-receiving portion and the first diffusion region; the method comprises forming the gate electrode on the first insulating film, which is located above the semiconductor substrate; implanting ions into the semiconductor substrate using the gate electrode as a mask to form the photo-receiving portion and a second diffusion region of the second conductivity type including a region that corresponds to the first diffusion region; forming a second insulating film above the semiconductor substrate; etching the second insulating film in a manner that the second insulating film remains above the photo-receiving portion and above a region of the substrate including an end of the second diffusion region near the gate electrode; and implanting ions as impurities of the second conductivity type into the second diffusion region using the remaining second insulating film as a mask to form the first diffusion region.
Conventional methods for manufacturing a solid-state imaging device employ plasma etching for etching the insulating film (etching the silicon oxide film 75 as in FIG. 11(c)). However, this leads to damage such as crystal defects, since the photo-receiving portion is directly exposed to the plasma, and such damage causes deterioration of the quality of the output image. Moreover, as has been mentioned before, the ion implantation for providing the photo-receiving portion with the same impurity concentration as the drain region (i.e. the detecting portion 74a in FIG. 9 and the charge drain portion 84a in FIG. 10) also inflicts damage upon the photo-receiving portion.
On the other hand, with the manufacturing method in accordance with the present invention, insulating film remains above the photo-receiving portion when the insulating film is etched, so that damage of the photo-receiving portion due to the etching can be avoided. Moreover, during the ion implantation for forming the first diffusion region, the insulating film covers the photo-receiving portion, so that damage of the photo-receiving portion due to the ion implantation can be reduced.
Another method for manufacturing a solid-state imaging device in accordance with the present invention comprises forming a gate electrode on a first insulating film, which is located above a semiconductor substrate having a first conductivity type; implanting ions into the semiconductor substrate using the gate electrode as a mask, to form a photo-receiving portion of a second conductivity type on a first side of the gate-electrode, and to form a second diffusion region of the second conductivity type on a second side of the gate-electrode; forming a second insulating film above the semiconductor substrate; etching the second insulating film in a manner that the second insulating film remains above the photo-receiving portion and above a region of the substrate including an end of the second diffusion region near the gate electrode; and implanting ions into the second diffusion region using the remaining second insulating film as a mask to form a first diffusion region with an impurity concentration that is higher than an impurity concentration in the photo-receiving portion. Also according to this manufacturing method, damage of the photo-receiving portion due to etching can be avoided, and damage of the photo-receiving portion due to ion implantation can be reduced.
It is preferable that the impurity concentration in the photo-receiving portion is adjusted to be lower than the impurity concentration in the second diffusion region. This can reduce the damage of the photo-receiving portion due to ion implantation even further.
It is preferable that the method further comprises forming a third diffusion region of the first conductivity type formed at an upper portion of a region in the semiconductor substrate that corresponds to the photo-receiving portion. This reduces dark currents, so that a good picture quality can be attained with more certainty.
It is preferable that forming the second insulating film is performed in a manner that the thickness of the second insulating film is 150 nm to 250 nm.
It is preferable that etching the second insulating film is performed by dry-etching.
It is preferable that the impurity concentration in the first diffusion region is at least 1020 cmxe2x88x923.
It is preferable that the impurity concentration in the photo-receiving portion is 1015 cmxe2x88x923 to 1019 cmxe2x88x923.
It is preferable that the impurity concentration in the second diffusion region is 1018 cmxe2x88x923 to 1019 cmxe2x88x923.